app: default app runner now runs the schedule once

crates/bevy_app/src/app.rs

@@ -1,17 +1,34 @@
 use crate::app_builder::AppBuilder;
 use bevy_ecs::{ParallelExecutor, Resources, Schedule, World};
 
-#[derive(Default)]
 pub struct App {
     pub world: World,
     pub resources: Resources,
-    pub runner: Option<Box<dyn Fn(App)>>,
+    pub runner: Box<dyn Fn(App)>,
     pub schedule: Schedule,
     pub executor: ParallelExecutor,
     pub startup_schedule: Schedule,
     pub startup_executor: ParallelExecutor,
 }
 
+impl Default for App {
+    fn default() -> Self {
+        Self {
+            world: Default::default(),
+            resources: Default::default(),
+            schedule: Default::default(),
+            executor: Default::default(),
+            startup_schedule: Default::default(),
+            startup_executor: Default::default(),
+            runner: Box::new(run_once),
+        }
+    }
+}
+
+fn run_once(mut app: App) {
+    app.update();
+}
+
 impl App {
     pub fn build() -> AppBuilder {
         AppBuilder::default()
@@ -30,9 +47,9 @@ impl App {
             &mut self.world,
             &mut self.resources,
         );
-        if let Some(run) = self.runner.take() {
-            run(self)
-        }
+
+        let runner = std::mem::replace(&mut self.runner, Box::new(run_once));
+        (runner)(self);
     }
 }
 

crates/bevy_app/src/app_builder.rs

@@ -202,7 +202,7 @@ impl AppBuilder {
     }
 
     pub fn set_runner(&mut self, run_fn: impl Fn(App) + 'static) -> &mut Self {
-        self.app.runner = Some(Box::new(run_fn));
+        self.app.runner = Box::new(run_fn);
         self
     }
 

examples/hello_world.rs

@@ -2,7 +2,6 @@ use bevy::prelude::*;
 
 fn main() {
     App::build()
-        .add_default_plugins()
         .add_system(hello_world_system.system())
         .run();
 }